The present invention relates to a damper assembly for closure of a gas duct and in particular is directed to a valve assembly for closure of a duct used for conveying hot, corrosive gas which carries a substantial quantity of dust.
In the oxidation of metal sulphides, e.g., by roasting, sulphur dioxide is evolved which may be used in the manufacture of sulphuric acid. Most of the dust in gas from a roaster is separated in a precipitator before the gas enters a manifold from which gas streams from more than one roaster are passed through scrubbing towers for further cleaning before entering the sulphuric acid making process. Periodically it is desirable to close off one duct to the manifold without interrupting the flow from other ducts and without causing the sulphur dioxide gas to escape to atmosphere or back up into the closed duct. However, since the gas is hot and corrosive and since it carries a substantial quantity of dust which is not removed by the precipitator, usual means of providing duct closure have proven to be ineffective.
In an operation in which a large rectangular cross section duct has been closed by inserting a metal damper plate transversely across the duct, corrosion and dust deposits around the plate have made it difficult to move the plate between open and shut positions. Presence of dust and corrosion products has made it difficult to obtain adequate seals, either between the upstream and downstream parts of the duct or between the interior of the dust and the outer atmosphere. Use of manually operated tools to force the damper plate into either open or closed positions has obviated effective sealing, and use of mechanical driving means has been encumbered by unbalanced forces resulting from uneven deposits of dust and corrosion products. Such unbalanced forces tend to buckle the damper plate.
U.S. Pat. No. 1,953,828 provides a horizontally acting valve which prevents gushing of oil from a well. The valve plate is guided by flanges on upper and lower casing plates as rotation of a threaded shaft moves the plate into open and closed positions.
U.S. Pat. No. 1,979,885 provides a vertically operating gate in a hot gas line. Spring diaphragms spaced about the conduit on one side of a gate plate act against mechanical means applied to the other side of said plate to provide a tight seal on closure and to release the plate for opening. Actuating mechanisms are isolated from the gas to protect them from heat and dust.
The body of the blind assembly of U.S. Pat. No. 2,354,967 for completely blanking off pipe lines has a transverse slot with machined faces. Either a flow controlling member or a blind having peripheral grooves to hold rubber sealing members is inserted into the slot. Flexible lips of the sealing members engage the machined faces of the body so that, with any difference in pressure between the inside and the outside, outer or inner lips will move to make a seal.
The apparatus of U.S. Pat. No. 2,702,178 has a valve plate which is held in position between two disc sealing members which encircle a gas conduit. Difficult operation of the control mechanism due to settling of dust is prevented by having one of the sealing members suspended about its section of conduit with sliding movement along the conduit being controlled by a guiding ring which is between the sealing member and the conduit. This ring and an outwardly bulging elastic member together prevent gas escape.
The slide valve assembly of U.S. Pat. No. 3,047,024 moves vertically between flanged sections of gas lines. The valve gate has a circular orifice to permit gas flow and a solid disc blind which are supported by three coplanar plate sections held together by edge strips. The circular orifice and the solid disc have peripheral grooves which are filled with gasket rings of rubber or with mixed rubber and asbestos to provide gastight seals when the valve is open or closed. Means are provided for exerting pressure on the seals in the direction of the gas line axis. The slide gate may be guided by side rollers mounted in the housing. The operating mechanism is isolated from dust in the gas stream.
The line blind of U.S. Pat. No. 3,099,292 discloses the use of a pivotal flat spectacle plate in combination with a pair of triangular body plates. Sealing is provided by O-rings set into peripheral grooves formed in the end surfaces of aligned pipe sections and wedging means are provided to exert pressure on the rings.
In U.S. Pat. No. 4,043,534, a sliding gate valve for a rectangular cross section gas duct is provided with side sealing means in the form of a resilient metal strip attached to the frame. The side edges of the gate make sealing contact with the spring strip, and slide along the strip when the gate is being closed. A flange mounted on the bottom of the frame and extending inwardly to contact the downstream surface of the gate when the gate is closed provides a bottom seal when upstream fluid pressure presses the gate against the flange. Top edge sealing may be provided by a similar flange or by flat sealing strips mounted on opposing sides of the aperture.
U.S. Pat. No. 4,093,245 provides tadpole tape sealing means at the bottom of a rectangular cross section gas duct. Resilience of the tadpole tape, backed by Z bar mounting, causes shaped shim stock strips to bear against a closed guillotine blade and to bear against each other when the blade is withdrawn.
The foregoing patents are generally directed to the application of mechanical or hydraulic forces along the axis of the conduit to provide tight sealing against stationary baffles and generally do not provide the maintenance of a gas tight seal as the valve plate is being moved between open and closed positions. When the damper plates of U.S. Pat. Nos. 4,043,534 and 4,093,245 are open, there is no protection of sealing means within the ducts against encrustation with solids deposited from the gas stream. Good sealing engagements on closure of the plates are not assured.
The damper assembly of the present invention provides sealing against gas leakage during movement between open and closed positions, provides protection of sealing surfaces against dust encrustation and does not require movement of structural parts towards the plate faces for the application of axial pressure necessary to maintain the seal in the open and closed positions. Other advantages will become apparent as the method and apparatus are described.